1. Technical Field
The present disclosure relates to assembly of an acoustic transducer, in particular a microelectromechanical (MEMS) capacitive microphone, to which the following description will make explicit reference, without this implying any loss of generality, and to a package for the assembly thus obtained.
2. Description of the Related Art
In a manner customary in this technical field, the term “package” will be used herein to indicate the casing or coating that surrounds, totally or partially, the die or dice of semiconductor material of the acoustic transducer, enabling electrical connection thereof from outside, for example, using the surface-mount (SMD) technique.
As is known, an acoustic transducer, for example a MEMS microphone of a capacitive type, generally comprises a MEMS sensing structure, designed to transduce acoustic-pressure waves into an electrical quantity (in particular, a capacitive variation), and a reading electronics, designed to execute appropriate processing operations (amongst which operations of amplification and filtering) of this electrical quantity so as to supply an electrical output signal (for example, an electrical voltage).
The MEMS sensing structure comprises in general a mobile electrode, formed as a diaphragm or membrane, set facing a fixed electrode, for providing the plates of a variable-capacitance sensing capacitor. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whilst a central portion thereof is free to move or bend in response to the pressure exerted by incident acoustic-pressure waves. The mobile electrode and the fixed electrode form a capacitor, and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance as a function of the acoustic signal to be detected.
In greater detail and with reference to FIG. 1, in a known embodiment, a microelectromechanical sensing structure 1 of a MEMS capacitive microphone comprises a membrane 2, which is mobile, is made of conductive material, and faces a back plate 3 (where by “back plate” is understood herein an element that is relatively rigid as compared to the membrane, which is, instead, flexible). The back plate 3 is formed by a first plate layer 3a, made of conductive material and facing the membrane 2, and by a second plate layer 3b, made of insulating material, set on the first plate layer 3a except for portions in which it extends through the first plate layer 3a to form protuberances, which start from the back plate 3 as a prolongation thereof towards the membrane 2 and have anti-stiction and stopper functions in regard to the movement of the membrane.
The membrane 2, which in use undergoes deformation as a function of the incident acoustic-pressure waves, is partially suspended above a substrate 5 and set directly facing a cavity 6, obtained by etching a rear surface 5b of the substrate 5 (opposite to a front surface 5a of the same substrate 5, set in the proximity of the membrane 2); the cavity 6 is defined as “back chamber” or “rear chamber”, in the case where the incident waves traverse the back plate 3 and has in this case the function of reference-pressure chamber. The membrane 2 is anchored to the substrate 5 by means of membrane anchorages 8, provided as protuberances of the membrane 2 that extend from its peripheral regions towards the same substrate. An insulation layer 9, for example made of silicon nitride (SiN), set on the substrate 5 enables, inter alia, electrical insulation of the membrane anchorages 8 from the substrate 5. The membrane anchorages 8 also have the function of suspending the membrane 2 over the substrate 5 at a certain distance therefrom. The value of this distance is a function of a compromise between the linearity of response at low frequencies and the noise of the MEMS capacitive microphone. In order to enable release of residual (tensile and/or compressive) stresses in the membrane 2, for example deriving from the manufacturing process, through openings 10 are formed through the membrane 2, in particular in the proximity of each membrane anchorage 8; the through openings 10 enable “equalization” of the static pressure on the two faces of the membrane 2.
The back plate 3 is anchored to the substrate 5 by means of plate anchorages 11 provided in its peripheral regions; the plate anchorages 11 are, for example, constituted by pillars made of the same conductive material as the back plate 3, set on top of the substrate 5 and electrically insulated from the substrate via the insulation layer 9. The back plate 3 rests peripherally on portions set on top of one another of a first sacrificial layer 12a, a second sacrificial layer 12b, and a third sacrificial layer 12c, external to the area occupied by the membrane 2 and by the plate anchorages 11. The back plate 3 also has a plurality of, preferably circular, holes 13 having the function of favouring, during the manufacturing steps, removal of the underlying sacrificial layers and, in use, enabling free circulation of air between the back plate 3 and the membrane 2 (making indeed the back plate 3 “acoustically transparent”). In use, the holes 13 consequently have the function of acoustic access port for enabling the acoustic-pressure waves to reach and deform the membrane 2.
Alternatively, in a way not illustrated in FIG. 1, the incident acoustic-pressure waves can reach the membrane 2 through the cavity 6, which hence performs, in this case, the function of acoustic access port (so-called “front chamber”).
The MEMS sensing structure 1 further comprises a membrane electrical contact 14 and a back-plate electrical contact 15, both made of conductive material, used, during operation of the MEMS microphone, for biasing the membrane 2 and the back plate 3 and collecting a capacitive-variation signal due to the deformation of the membrane 2 caused by the incident acoustic-pressure waves. As illustrated in FIG. 1, the membrane electrical contact 14 is formed in part in the same layer in which the back plate 3 is provided, from which it is electrically insulated, and is electrically connected to the membrane 2 via a conductive plug. The back-plate electrical contact 15 can be advantageously provided in the same layer in which the back plate 3 is provided, by contacting it directly, and is electrically connected to a contact pad accessible from outside.
In a known way, the sensitivity of the MEMS capacitive microphone depends upon the mechanical characteristics of the membrane 2 of the MEMS sensing structure 1 (in particular, upon its mechanical compliance), and moreover upon assembly of the membrane 2 and of the back plate 3. Furthermore, the volume of the front acoustic chamber (traversed in use by the incident acoustic-pressure waves) and back acoustic chamber (set at the reference pressure) has a direct effect on the acoustic performance. In particular, the volume of the front chamber determines in a known way the upper resonance frequency of the microphone, and hence its performance for high frequencies (the front chamber constitutes in fact a sort of Helmholtz resonator): in general, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the microphone. In a known way, it is also desirable to provide a back chamber of large dimensions so as to improve the frequency response and sensitivity of the microphone.
The package of the microphone must moreover be configured so as to house not only the MEMS sensing structure 1 but also the reading electronics associated thereto, generally provided as an ASIC (Application Specific Integrated Circuit), electrically coupled to the MEMS sensing structure 1. At the design stage, it must also be considered that acoustic transducers typically operate in unfavourable working environments, for example ones subject to high levels of RF radiation (when integrated in mobile phones, or similar wireless-communication devices).
The presence of acoustic access ports, directly communicating with the external environment, designed to enable passage of the acoustic-pressure waves towards the membrane 2, leads to the further requirement of providing appropriate screens for the incident light and barriers for the particles of dust or other material, which could jeopardize proper operation of the MEMS sensing structure and of the reading electronics.
Therefore, there is a wide range of constraints imposed upon assembly of a capacitive MEMS microphone and upon the package for the resulting assembly, which render design thereof particularly problematical, in particular where very low size is required.
A solution of assembly that has been proposed (see, for example, the patent application No. WO 2007/112743) envisages providing in a single die of semiconductor material (for example, silicon) both the MEMS sensing structure and the reading electronics of the MEMS capacitive microphone. This assembly solution is, however, very complex, posing various problems of technological compatibility. In fact, it is known that the methods and techniques for manufacturing of MEMS sensing structures differ sensibly from those of integrated electronic circuits.
An alternative solution of assembly hence envisages provision of two distinct dice of semiconductor material, a first die for the MEMS sensing structure and a second die for the reading circuitry. In a solution of this type, illustrated schematically in FIG. 2 (and described, for example, in the U.S. Pat. No. 6,781,231), a first die 20, integrating the MEMS sensing structure (illustrated schematically), and a second die 21, integrating an ASIC of the reading electronics, are coupled side-by-side on a substrate 22 of a corresponding package 24. Electrical connections 25 between the first and second dice 21, 22 are made using the wire-bonding technique, whilst appropriate metal layers and vias (not illustrated in detail) are provided in the substrate 22 for routing the electrical signals outside the package 24. Moreover, a cover 26 of the package 24 is coupled to the substrate 22, enclosing within it the first and second dice 21, 22; the cover 26 can be made of metal or pre-moulded plastic with metal layers such as to prevent disturbance caused by external electromagnetic signals (by providing a sort of Faraday cage). The cover 26 also has an opening 28 to enable entry of acoustic pressure waves; advantageously, a screen (not illustrated) may be coupled to the opening 28 for screening the incident light, or else a filter (not illustrated) to prevent access within the cover 26 of particles of dust or other material. Alternatively, or in addition, a protective coating 29, for example made of resin, can be set on the second die 22 so as to function as a further protection for the reading electronics in regard to the incident light and impurities. Pads (not illustrated) are provided on the bottom side of the substrate 22 for soldering and electrical connection to an external printed circuit board.
Also this solution is not, however, free from drawbacks, amongst which the fact of entailing large dimensions for accommodating side-by-side the two dice of the acoustic transducer and for providing the corresponding package. Furthermore, this solution fails to offer the designer an ample freedom (as instead would be desirable) in dimensioning of the chambers of the acoustic transducer, for determination of its electrical characteristics.
A further solution of assembly that has been proposed (see, for example, the U.S. Pat. No. 6,088,463) envisages assembly of the two dice of the MEMS sensing structure and of the reading electronics on a third die of semiconductor material, distinct from the first two dice, in which sensing structures or electronic circuits are not present, but which performs only the function of mechanical support (a so-called “dummy” die). The dice of the reading electronics and of the MEMS sensing structure are electrically and mechanically connected to the supporting die using the bump-bonding technique. In one embodiment, the two dice of the MEMS sensing structure and of the reading electronics are coupled to opposite external faces of the supporting die, and the supporting die is hence set between them. A coating cover encapsulates the entire structure, except for an access opening provided in the supporting die; this access opening communicates with the membrane of the MEMS sensing structure through a cavity etched in the same supporting die.
A solution of this type is complex to implement in so far as it entails laborious procedures of bonding with the further supporting die and of machining of the same supporting die, and is not free from problems linked to the required dimensions, which are large both in a lateral direction and in a vertical direction. Furthermore, the dimensions of the front and back chambers of the acoustic transducer are defined in a fixed way by the dimensions of the supporting die, and consequently pose constraints on sizing of the microphone performance.
Consequently, the need is certainly felt in this field to provide a suitable assembly for an acoustic transducer, in particular a MEMS capacitive microphone, and a corresponding package, which will enable the features previously referred to of low manufacturing costs, high acoustic performance and reliability, and small dimensions (comparable, for example, to those of the so-called “micro-SMD” packages) to be achieved.